(1) Field of the Invention
The present application stems from French patent application FR 11/01327 filed on Apr. 29, 2011, the content of which is incorporated herein.
The invention relates to the general field of regulating the engines of an aircraft, in particular of a rotary wing aircraft.
(2) Description of Related Art
This type of aircraft generally has at least one rotor for providing lift and possibly also propulsion, which rotor has a plurality of variable-pitch blades, and the aircraft also has a power plant of the type comprising an engine driving the main rotor in rotation by means of a main gearbox known under the acronym MGB.
Such an engine may be a free turbine turboshaft engine having a gas generator followed by at least one free turbine that is secured to an outlet shaft of the engine, the outlet shaft being suitable for driving the main gearbox of an aircraft.
Furthermore, one overrunning clutch or “free-wheel” per engine is generally arranged between the engine and the main gearbox, in particular for the purpose of preventing a mechanical blockage of the engine giving rise to a blockage of the main gearbox and consequently to a blockage of the main rotor of the rotary wing.
Conventionally, the free-wheel thus has a driving portion suitable for rotating a driven portion of the free-wheel. The driving portion of the free-wheel is then connected to the engine, while the driven portion of the free-wheel is connected to the main gearbox.
The term “synchronization” is used to designate a first operating state in which the engine drives the rotary wing, the driving portion of the free-wheel engaging the driven portion of the free-wheel.
Conversely, the term “desynchronization” relates to a second operating state in which the engine is not driving the rotary wing, the driven portion of the free-wheel not engaging the driving portion of the free-wheel.
In another aspect, the engine is fitted with a regulator system having the main function of using the fuel flow rate to control the power that is delivered by the engine. The regulator system may seek to maintain the speed of rotation of the main rotor of a helicopter at a value that is substantially constant, by maintaining the speed of rotation of a gas generator of the engine below a predetermined threshold, for example.
A helicopter is piloted in particular by acting on the pitch of the blades of the main rotor.
An increase in pitch tends to cause the main rotor to decelerate, the increase in pitch giving rise to an increase in the lift from the main rotor and also to an increase in its aerodynamic drag. In order to avoid the speed of rotation being reduced excessively, the power generated by the engine must therefore increase so as to keep the speed of rotation of the main rotor at the desired speed. Similarly, when the pitch of the blades is decreased, it is necessary to decrease the power delivered by the engine so that the speed of rotation of the main rotor does not exceed the limit set by the manufacturer.
Furthermore, the regulation of the fuel flow rate must be controlled scrupulously since any increase in the fuel flow rate must be controlled in particular to enable the rotary wing to absorb more power, but without running the risk of the engine pumping. “Pumping” is a phenomenon that affects the gas generator of an engine when, locally, an excessive angle of incidence of a blade or a vane gives rise to aerodynamic separation, thereby considerably reducing the flow rate of air. One consequence of this phenomenon is overheating in the combustion chamber of the engine, which can lead to damage to a turbine of the engine.
Likewise, decelerating the engine and reducing the fuel flow rate must also be controlled so as to avoid engine flameout.
For example, on a helicopter there are various types of regulation with the purpose of constantly bringing the speed of rotation of the free turbine towards a predefined first setpoint value.
One known type of regulation is proportional-integral regulation with power anticipation based on the collective pitch of the blades of the main rotor. That type of regulation seeks to maintain the speed of rotation of the free turbine substantially equal to the regulation first setpoint value. Under such circumstances, a computer makes use of information from sensors for sensing the speed of rotation of the free turbine.
For example, sensors measure the first speed of rotation NTL of the free turbine of the engine. Under such circumstances, a first setpoint value, corresponding to the value that the first speed of rotation NTL ought to have in order to ensure that the speed of rotation of the rotor is ideal, is a value that is set by the manufacturer.
As a result, if the first speed of rotation NTL is different from this first setpoint value, the computer accelerates or decelerates the engine in order to obtain the ideal speed of rotation for the main rotor.
Under such conditions, the computer determines a second setpoint value by making use of anticipation relationships based on the collective pitch of the blades of the main rotor. This second setpoint value corresponds to the value that the second speed of rotation NG of the gas generator of the engine ought to reach in order to ensure that the first speed of rotation NTL is equal to the first setpoint value.
Reference may be made to the literature in order to obtain information about proportional-integral regulation with power anticipation.
The present invention relates more particularly to regulating transients in such an engine in the event of the pilot performing a severe maneuver, i.e. in the event of a fast and substantial variation in the collective pitch of the blades.
Following a large and fast drop in the collective pitch, the speed of rotation of the rotary wing increases significantly. Under such circumstances, the engine and the main gearbox lose synchronization during a transient period of autorotation, with the corresponding speed of rotation of the driving portion of the free-wheel becoming less than the speed of rotation of the driven portion of the free-wheel.
However, when the pilot increases the collective pitch once more, power anticipation enables the engine to be accelerated so as to respond in the most satisfactory possible manner to the power requirement of the main rotor.
Conversely, the third speed of rotation NR of the main rotor drops suddenly as a result of the increase in collective pitch, thereby leading to an increase in the drag of said main rotor.
As a result, the speed of rotation of the driving portion of the free-wheel arranged between the engine and the main gearbox increases, whereas on the contrary the speed of rotation of the driven portion of the free-wheel drops.
The first state of operation corresponding to synchronized operation is finally achieved when the driven portion engages the driving portion. The helicopter then ceases to be flying in autorotation.
Nevertheless, the resynchronization takes place suddenly. The free-wheel is subjected to an impact at the moment of resynchronization, associated with the acceleration difference between the main rotor and the free turbine of the engine.
Furthermore, it should be observed that the third speed of rotation of rotary wing drops below its nominal value for the length of time needed by the engine to return to delivering sufficient power to enable it to drive the rotary wing.
Among known types of regulation, mention may be made of several documents.
Document EP 0 093 684 describes making use of the difference between the speed of rotation of the main rotor and the speed of rotation of the free turbine in order to determine when there is a state of autorotation. During such autorotation, the deceleration of the main rotor is used for controlling the flow rate of fuel to the engine.
By way of example, document U.S. Pat. No. 5,046,923 describes regulation that is based on an algorithm that takes account of the speed of rotation of the main rotor, of the speed of rotation of the engine, and, where appropriate, of the engine torque, in order to determine when there is a state of autorotation. Once the above-mentioned autorotation state is recognized, a speed of rotation setpoint for the gas generator of the engine is calculated, as is a setpoint for the fuel flow rate.
Document EP 0 398 839 describes automatic control of the fuel feed to a free turbine helicopter engine. By comparing the speed of the main rotor with the speed of the free turbine, various setpoints are applied. Taking account of the acceleration of the rotor serves to control an anticipation setpoint for the speed NG (where NG is the speed of rotation of the gas generator). Mention is also made of the possibility of also making use of the torque delivered by the engine.